Liquid crystalline materials exhibiting the cholesteric mesophase have been proposed for use in a variety of optical applications, for example, notch filters, circular polarizing filters, selective reflectors, beam splitters, and beam apodizers. U.S. Pat. No. 3,711,181, for example, discloses an optical apparatus for modulating circular-polarized light that contains optically negative liquid crystal films.
It has been recognized that low molecular weight liquid crystal materials suffer limitations as to durability, effective temperature, mesophase stability, and amenability to processes for device fabrication. Polymeric liquid crystalline compositions have been proposed as potentially useful for overcoming such limitations. U.S. Pat. No. 4,293,435 discloses a liquid-crystalline cholesteric polymer phase that consists essentially of a copolymer of particular nematogenic and chiral acrylic ester monomers. In U.S. Pat. No. 4,410,570 is disclosed a liquid crystalline phase that contains a cyclic organopolysiloxane to which is chemically bonded at least one mesogenic group. Thermotropic cholesteric liquid crystalline glutamate copolymers consisting of chiral glutamate ester repeating units are disclosed in U.S. Pat. No. 4,743,675.
Many applications of chiral liquid crystalline polymeric materials in optical devices require polymers capable of forming both right- and left-handed helical structures. When a film of such a polymer is applied to a substrate, the helical structures must be capable of forming and maintaining the Grandjean texture, in which the helical axis is perpendicular to the substrate surface, to enable the selective reflection of circular-polarized light. An enantiomeric chiral pair of liquid crystalline polymers, whose individual structures are characterized as a right-handed and a left-handed helix, are thus capable of selectively reflecting fight-handed and left-handed circular-polarized light, respectively.
Especially useful for optical information storage applications are chiral nematic liquid crystalline polymers which form clear, transparent films that absorb no light in the visible region but do selectively reflect visible circular-polarized light. It is especially desirable for the application of these films as high efficiency polarizers that the reflected light be characterized by a broad half band width (HBW), as defined by the width of the spectral band measured at one-half of its maximum height. In devising flat panel displays, for example, where electrical power requirements should be kept as low as possible, chiral nematic liquid crystalline copolymers that form films whose half band widths in the visible region are substantially broader than those known in the art would be extremely useful.
In addition to the just described optical characteristics, it is necessary that the chiral nematic polymers be readily synthesized and that they have solubility properties which enable their processing into clear glassy thin films for use as optical devices. All of these requirements are met by the chiral nematic liquid crystalline copolymer compositions of the present invention.